1. Field of the Invention
The present invention relates to a semiconductor device, and in particular, to a thin film transistor and a method of fabricating the thin film transistor.
2. Background of the Related Art
Thin film transistors serve as switching devices switching image data signals in each pixel region. Thin film transistors can be used instead of CMOS load transistors or load resistors of a static random access memory (SRAM) of more than 1M bit. A liquid crystal display (LCD) includes an upper glass, a lower glass, and a liquid crystal interposed between the upper and lower glasses. The upper glass has a black matrix layer, a common electrode, and R, G and B color filter layers. The lower glass has data lines and gate line; crossing each other and pixel regions arranged in matrix. A pixel electrode is formed in each pixel region, and an amorphous thin film transistor acting like an analog switch is formed to control charge stored in its capacitor.
FIG. 1 is a lay-out of a related art liquid crystal display. As shown in FIG. 1, the lower glass includes a plurality of scanning lines 11 formed extending in one direction, a gate electrode 11a extending from each scanning line 11 and data lines 12 crossing the scanning lines 11. A thin film transistor includes a source electrode 12a and a drain electrode 12b extending from the data lines 12.
Black matrix layers (not shown) are arranged on the upper glass like a gauze to shut out the light in a region except the pixel electrodes (not shown) formed on the lower glass. R, G and B color filter layers (not shown) are formed between the black matrix layers. Further, a common electrode (not shown) is formed over the color filter layers and black matrix layers.
As shown in FIG. 2, a related art thin film transistor includes a gate electrode 11a formed on an insulating substrate 21, a gate insulating layer 22 disposed on gate electrode 11a and an amorphous silicon layer 23 disposed on gate insulating layer 22 to enclose the gate electrode 11a. An n+ silicon layer is formed as an ohmic layer 24 to expose a part of the amorphous silicon layer 23 on the gate electrode 11a, and the source electrode 12a and the drain electrode 12b are formed on the ohmic layer 24. The material of each of the source and drain electrodes 12a and 12b is molybdenum.
The process of manufacturing a related art thin film transistor will now be described. As shown in FIG. 3A, the gate electrode 11a is formed on a predetermined area of the insulating substrate 21. An insulating layer such as siliconitride SiN is formed on the substrate 21 including the gate electrode 11a to form the gate insulating layer 22. The insulating material used as the gate insulating layer 22 serves as a capacitor dielectric in a storage capacitor area. As shown in FIG. 3B, an amorphous silicon layer 32 and an n+ silicon layer 33 are formed on the gate insulating layer 22.
As shown in FIG. 3C, the n+ silicon layer 33 and the amorphous silicon layer 32 are selectively removed to enclose the gate electrode 11a. Molybdenum (Mo) is applied all over substrate 21 including the n+ silicon layer 33 as source and drain electrodes. The molybdenum material forming the source and drain electrodes and the n+ silicon layer 33 are serially etched to expose the amorphous silicon layer 32 corresponding to a channel region and form the source and drain electrodes 12a and 12b. Molybdenum (not shown), the material forming the source and drain electrodes, is patterned on the gate insulating layer 22 in the storage capacitor area of the pixel region to contact a pixel electrode in the post manufacturing process.
As shown in FIG. 3D, a passivation layer 34 is formed all over the substrate 21 including the source and drain electrodes 12a and 12b. Thus, the manufacture of the thin film transistor is complete.
In the manufacture of the thin film transistor, when fluorine (F) gas is used as an etching gas in an etching process to form source and drain electrodes, an etching selection ratio of n+ silicon layer 33 and amorphous silicon layer 32 cannot be secured. Thus, Chlorine (Cl) gas is used instead of the fluorine gas to solve the above problem. When using Cl gas, there is no etching selection ratio with gate insulating layer 22 of the storage capacitor area, which causes excessive etching of gate insulating layer 22, and, what is worse, gate insulating layer 22 may be opened.
A second related art thin film transistor was proposed to solve the above problem, and a method of fabricating the second related art thin film transistor will now be described. FIGS. 4A to 4J depict the steps in the manufacture of the second related art thin film transistor.
As shown in FIG. 4A, a gate material 44 formed of chromium 42 and molybdenum 43 is formed on a predetermined area of substrate 41. The gate material 44 can be constituted by either two layers (e.g., chromium 42 and molybdenum 43) or one layer.
Referring to FIG. 4B, the gate material 44 is patterned to form a gate electrode 44a by a general patterning process. Sides of the gate electrode 44a have a slant when formed by a reactive ion etching A) when patterning the two-layered gate electrode 44a formed by molybdenum 43 and chromium 42.
As shown in FIG. 4C, after patterning the gate electrode 44a, a gate insulating layer 45 is deposited all over the gate electrode 44a and the substrate 41. The gate electrode 44a is inclined at its edge to improve the coverage in the corresponding area. A gate electrode having a slant at its edge and a technique of improving the coverage are disclosed in U.S. Pat. No. 5,132,745.
As shown in FIG. 4D, an amorphous silicon layer 46 is serially deposited on the gate insulating layer 45 in a vacuum chamber. An n+ amorphous silicon layer 47 is serially deposited on the amorphous silicon layer 46. Subsequently, as shown in FIG. 4E, the n+ amorphous silicon layer 47 and the amorphous silicon layer 46 are selectively removed except in an area where a thin film transistor is formed on the substrate 41.
As shown in FIG. 4F, a first conductive layer 48 is deposited to a thickness of 0.01 to 0.1 .mu.m on the gate insulating layer 45 including the patterned n+ amorphous silicon layer 47 and the amorphous silicon layer 46. The first conductive layer 48 is made of chromium (Cr) in ohmic contact with the n+ amorphous silicon layer 47. The first conductive layer 48 can be made of a material such as nichromium (nickel and chrormium) and tantalum.
A second conductive layer 49 is deposited to a thickness of 0.1 to 1 .mu.m on the first conductive layer 48. Thus, the second conductive layer 49 is relatively larger than the first conductive layer 48. The second conductive layer 49 is made of molybdenum, and may be made of aluminum or tungsten. The use of molybdenum as second conductive layer 49 assures better conductivity than that of source and drain electrodes made of chromium (Cr), which constitutes the first conductive layer 48. Molybdenum assures a good ohmic contact for source and drain electrodes and the n+ amorphous silicon layer 47.
As shown in FIG. 4G, a photoresist 50 is applied on the second conductive material 49. The photoresist 50 corresponding to the channel region of the thin film transistor is removed, and the photoresist 50 is patterned to have edges slanted by 45.degree..
As shown in FIG. 4H, the second conductive material 49 is etched using the photoresist 50 as a mask with a requirement that the first conductive layer 48 is not effected. Based on the requirement of the etching process, SF.sub.6 gas of 37.5 sccm, Cl.sub.2 gas of 6.5 sccm, and O.sub.2 gas of 16 sccm are used and a pressure of 6.5 mTorr is maintained. The etching process is carried out under Rf plasma. Since the photoresist 50 is patterned with inclined edges, the second conductive material 49 also becomes patterned with inclined edges.
As shown in FIG. 4I, the exposed first conductive layer 48, Cr, is selectively etched by changing the requirements for etching the second conductive layer 49. In other words, the first conductive layer 48 is etched in a different process than the second conductive layer 49. Etching the first conductive layer 48 forms a source electrode 51 and a drain electrode 51a, which are respectively made of the first conductive layer 48 and second conductive layer 49.
The first conductive layer 48 is etched away by the use of chroline gas, Cl.sub.2 gas of 70 sccm and O.sub.2 gas of 30 sccm as a source gas under the pressure of 100 mTorr. The first conductive material 48 and the photoresist are in the etching ratio of 1 to 1. Thus, the first conductive layer 48, Cr, is used as an etch stopper of the second conductive layer 49, and the etching speed of molybdenum, the second conductive layer 49 is higher than that of first conductive layer 48, Cr. When etching molybdenum and chromium as the source and drain electrodes 51 and 51a, molybdenum and chromium are inclined at their respective edges. As shown in FIG. 4J, the exposed n+ amorphous silicon layer 47 is etched to expose a part of the amorphous silicon layer 46. Further, after removing the photoresist, a the passivation layer 60 is formed all over the substrate 41 including the source electrode 51 and the drain electrode 51a, to complete the manufacture of the second related art thin film transistor.
The related art method of manufacturing the thin film transistor has various problems. When etching the source and drain electrodes made of molybdenum, the first conductive material, and chromium, the second conductive material, a two-step etching process is needed that increases the time to perform the etching process. Further, when etching molybdenum, the first conductive material, if the chromium, which is used as an etch stopper, is not uniformly deposited and has holes, the etching gas for etching the molybdenum etches the n+ amorphous silicon through the holes. Thus, an open in a signal line or driving degradation of thin film transistors can occur.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.